Light redirecting film and display system incorporating same

ABSTRACT

A light redirecting film ( 100 ) includes a first major surface ( 110 ) that includes first microstructures ( 150 ) that extend along a first direction and a second major surface ( 120 ), which may form part of a matte layer, the second major surface being opposite to the first major surface and including second microstructures ( 160 ). The second major surface has an optical haze that is not greater than about 3% and an optical clarity that is not greater than about 85%. The light redirecting film has an average effective transmission that is not less than about 1.75. The light redirecting film ( 100 ) may comprise particles. The second microstructures may have a slope distribution.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US2010/036018, filed on May 25, 2010, which claims priority to U.S.Provisional Application No. 61/183,154, filed on Jun. 2, 2009, thedisclosure of which is incorporated by reference in its/their entiretyherein.

FIELD OF THE INVENTION

This invention generally relates to optical films for redirecting light.The invention is further applicable to optical systems, such as displaysystems, incorporating such optical films.

BACKGROUND

Display systems, such as liquid crystal display (LCD) systems, are usedin a variety of applications and commercially available devices such as,for example, computer monitors, personal digital assistants (PDAs),mobile phones, miniature music players, and thin LCD televisions. MostLCDs include a liquid crystal panel and an extended area light source,often referred to as a backlight, for illuminating the liquid crystalpanel. Backlights typically include one or more lamps and a number oflight management films such as, for example, lightguides, mirror films,light redirecting films, retarder films, light polarizing films, anddiffuser films. Diffuser films are typically included to hide opticaldefects and improve the brightness uniformity of the light emitted bythe backlight.

SUMMARY OF THE INVENTION

Generally, the present invention relates to light redirecting films. Inone embodiment, a light redirecting film includes a first major surfacethat includes a plurality of first microstructures that extend along afirst direction. The light redirecting film also includes a second majorsurface that is opposite to the first major surface and includes aplurality of second microstructures. The second major surface has anoptical haze that is not greater than about 3% and an optical claritythat is not greater than about 85%. The light redirecting film has anaverage effective transmission that is not less than about 1.75. In somecases, the plurality of first microstructures include a plurality oflinear prisms that extend along the first direction. In some cases, themaximum height of a microstructure in the plurality of firstmicrostructures is different than the maximum height of anothermicrostructure in the plurality of first microstructures. In some cases,the height of a microstructure in the plurality of first microstructuresvaries along the first direction. The plurality of secondmicrostructures can include protrusions and/or recessions. In somecases, the plurality of second microstructures covers at least about80%, or at least about 85%, or at least about 90%, or at least about95%, of the second major surface. The plurality of secondmicrostructures can form regular or irregular patterns. In some cases,no more than about 7%, or no more than about 5%, or no more than about3%, of the second major surface has a slope magnitude that is greaterthan about 3.5 degrees. In some cases, no more than about 4%, or no morethan about 2%, or no more than about 1%, of the second major surface hasa slope magnitude that is greater than about 5 degrees. In some cases,the second microstructures are not formed primarily by any particlesthat the light redirecting film may include. In some cases, the lightredirecting film does not include particles that have an average sizegreater than about 0.5 microns. In some cases, the microstructures inthe plurality of second microstructures have a slope distribution thathas a half width half maximum (HWHM) that is not greater than about 6degrees. In some cases, the light redirecting film includes a substratelayer that has opposing first and second major surfaces, a first layerthat is disposed on the first major surface of the substrate layer andincludes the first major surface of the light redirecting film, and amatte layer that is disposed on the second major surface of thesubstrate layer and includes the second major surface of the lightredirecting layer. In some cases, the first layer has an index ofrefraction that is not less than about 1.6. IN some cases, the mattelayer includes a plurality of particles that have an average size thatis less than the average size of the plurality of second microstructuresby at least a factor of 5. In some cases, if the matte layer includesparticles, then the average thickness of the matte layer is at least 2microns greater than the average size of the particles. In some cases,if the matte layer includes particles, then the average thickness of thematte layer is greater than the average size of the particles by atleast a factor of 2.

In another embodiment, a light redirecting film includes a first majorsurface that includes a plurality of linear microstructures, and asecond major surface that is opposite to the first major surface andincludes a plurality of second microstructures. The second major surfacehas an optical haze that is not greater than about 3% and an opticalclarity that is not greater than about 85%. The average effectivetransmission of the light redirecting film is not less, or is less by nomore, than about 1.5% as compared to a light redirecting film that hasthe same construction except for comprising a smooth second majorsurface. In some cases, the plurality of second microstructures hasgeometrical symmetry and asymmetric slope distribution. In some cases,the plurality of second microstructures includes geometrical asymmetryand symmetric slope distribution.

In another embodiment, an optical stack includes a first lightredirecting film that includes a first major surface and an opposingsecond major surface, where the first major surface includes a firstplurality of microstructures that extend along a first direction, andthe second major surface includes a second plurality of microstructures.The optical stack also includes a second light redirecting film thatincludes a third major surface and an opposing fourth major surface,where the third major surface faces the second major surface of thefirst light redirecting film and includes a third plurality ofmicrostructures that extend along a second direction that is differentthan the first direction, and the fourth major surface includes a fourthplurality of microstructures. Each of the second and fourth majorsurfaces has an optical haze that is not greater than about 3% and anoptical clarity that is not greater than about 85%. In some cases, theoptical stack has an average effective transmission that is not lessthan about 2.5.

In another embodiment, an optical stack includes a first lightredirecting film that includes a first major surface and an opposingsecond major surface, where the first major surface includes a firstplurality of microstructures that extend along a first direction. Theoptical stack also includes a second light redirecting film thatincludes a third major surface and an opposing fourth major surface,where the third major surface faces the second major surface of thefirst light redirecting film and includes a third plurality ofmicrostructures that extend along a second direction that is differentthan the first direction. Each of the second and fourth major surfaceshas an optical haze that is not greater than about 3% and an opticalclarity that is not greater than about 85%. The average effectivetransmission of the optical stack is not less or is less by no more thanabout 1% as compared to an optical stack that has the same constructionexcept for including smooth second and fourth major surfaces. In somecases, the optical stack has an average effective transmission that isnot less as compared to an optical stack that has the same constructionexcept for including smooth second and fourth major surfaces.

In another embodiment, an optical film includes a structured majorsurface that has geometrical symmetry and asymmetric slope distribution.In some cases, the optical film has an optical haze that is not greaterthan about 3% and an optical clarity that is not greater than about 85%.

In another embodiment, an optical film includes a structured majorsurface that has geometrical asymmetry and symmetric slope distribution.In some cases, the optical film has an optical haze that is not greaterthan about 3% and an optical clarity that is not greater than about 85%.

BRIEF DESCRIPTION OF DRAWINGS

The invention may be more completely understood and appreciated inconsideration of the following detailed description of variousembodiments of the invention in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic side-view of a light redirecting film;

FIG. 2 is a schematic side-view of an optical system for measuringeffective transmission;

FIG. 3A is a schematic side-view of recessed microstructures;

FIG. 3B is a schematic side-view of protruding microstructures;

FIG. 4A is a schematic top-view of regularly arranged microstructures;

FIG. 4B is a schematic top-view of irregularly arranged microstructures;

FIG. 5 is a schematic side-view of a microstructure;

FIG. 6 is calculated optical haze versus surface fraction “f”;

FIG. 7 is calculated optical clarity versus surface fraction “f”;

FIG. 8 is a schematic side-view of an optical film;

FIG. 9 is a schematic side-view of another optical film;

FIG. 10 is a schematic side-view of a cutting tool system;

FIGS. 11A-11D are schematic side-views of various cutters;

FIGS. 12-17 are optical micrographs of various microstructured surfaces;

FIGS. 18-20 are SEM images of various microstructured surfaces atdifferent magnifications;

FIG. 21 is an AFM surface profile of a microstructured surface;

FIG. 22A-22B are cross-sectional profiles of the microstructured surfacein FIG. 21 along two mutually orthogonal directions;

FIG. 23 is percent slope distribution for the microstructured surface inFIG. 21 along two mutually orthogonal directions;

FIG. 24 is percent height distribution for the microstructured surfacein FIG. 21;

FIG. 25 is percent slope magnitude distribution for the microstructuredsurface in FIG. 21;

FIG. 26 is percent cumulative slope distribution for the microstructuredsurface in FIG. 21;

FIG. 27 is percent cumulative slope distributions for variousmicrostructured surfaces;

FIG. 28 is a schematic side-view of an optical stack; and

FIG. 29 is a schematic side-view of a display system.

In the specification, a same reference numeral used in multiple figuresrefers to the same or similar elements having the same or similarproperties and functionalities.

DETAILED DESCRIPTION

This invention generally relates to light redirecting films that arecapable of substantially enhancing brightness in a display system whileat the same time masking and/or eliminating physical defects such asscratches, and undesirable optical effects such as moiré and color mura.The disclosed light redirecting films include an array of linearmicrostructures for improving brightness and a matte surface forimproving the display cosmetics. The optical haze of the matte surfaceis sufficiently low to maintain brightness, and the optical clarity ofthe matte surface is sufficiently low to mask and/or eliminate defects.

FIG. 1 is a schematic side-view of a light redirecting film 100 forredirecting an incident light toward a desired direction. Lightredirecting film 100 includes a first major surface 110 that includes aplurality of microstructures 150 that extend along the y-direction.Light redirecting film 100 also includes a second major surface 120 thatis opposite first major surface 110 and includes a plurality ofmicrostructures 160.

Light redirecting film 100 also includes a substrate layer 170 that isdisposed between respective first and second major surfaces 110 and 120and includes a first major surface 172 and an opposing second majorsurface 174. Light redirecting film 100 also includes a prism layer 130that is disposed on first major surface 172 of the substrate layer andincludes first major surface 110 of the light redirecting film, and amatte layer 140 that is disposed on second major surface 174 of thesubstrate layer and includes second major surface 120 of the lightredirecting film. The matte layer has a major surface 142 opposite majorsurface 120.

The exemplary light redirecting film 100 includes three layers 130, 170and 140. In general, the light redirecting film can have one or morelayers. For example, in some cases, the light redirecting film can havea single layer that includes respective first and second major surfaces110 and 120. As another example, in some cases, the light redirectingfilm can have many layers. For example, in such cases, substrate 170 canhave multiple layers.

Microstructures 150 are primarily designed to redirect light that isincident on major surface 120 of the light redirecting film, along adesired direction, such as along the positive z-direction. In theexemplary light redirecting film 100, microstructures 150 are prismaticlinear structures. In general, microstructures 150 can be any typemicrostructures that are capable of redirecting light by, for example,refracting a portion of an incident light and recycling a differentportion of the incident light. For example, the cross-sectional profilesof microstructures 150 can be or include curved and/or piece-wise linearportions. For example, in some cases, microstructures 150 can be linearcylindrical lenses extending along the y-direction.

Each linear prismatic microstructure 150 includes an apex angle 152 anda height 154 measured from a common reference plane such as, forexample, major plane surface 172. In some cases, such as when it isdesirable to reduce optical coupling or wet-out and/or improvedurability of the light redirecting film, the height of a prismaticmicrostructure 150 can change along the y-direction. For example, theprism height of prismatic linear microstructure 151 varies along they-direction. In such cases, prismatic microstructure 151 has a localheight that varies along the y-direction, a maximum height 155, and anaverage height. In some cases, a prismatic linear microstructure, suchas linear microstructure 153, has a constant height along they-direction. In such cases, the microstructure has a constant localheight that is equal to the maximum height and the average height.

In some cases, such as when it is desirable to reduce optical couplingor wet-out, some of the linear microstructures are shorter and some ofthe linear microstructures are taller. For example, height 156 of linearmicrostructure 153 is smaller than height 158 of linear microstructure157.

Apex or dihedral angle 152 can have any value that may be desirable inan application. For example, in some cases, apex angle 152 can be in arange from about 70 degrees to about 110 degrees, or from about 80degrees to about 100 degrees, or from about 85 degrees to about 95degrees. In some cases, microstructures 150 have equal apex angles whichcan, for example, be in a range from about 88 or 89 degree to about 92or 91 degrees, such as 90 degrees.

Prism layer 130 can have any index of refraction that may be desirablein an application. For example, in some cases, the index of refractionof the prism layer is in a range from about 1.4 to about 1.8, or fromabout 1.5 to about 1.8, or from about 1.5 to about 1.7. In some cases,the index of refraction of the prism layer is not less than about 1.5,or not less than about 1.55, or not less than about 1.6, or not lessthan about 1.65, or not less than about 1.7.

In some cases, such as when light redirecting film 100 is used in aliquid crystal display system, the light redirecting film 100 canincrease or improve the brightness of the display. In such cases, thelight redirecting film has an effective transmission or relative gainthat is greater than 1. As used herein, effective transmission is theratio of the luminance of the display system with the film in place inthe display system to the luminance of the display without the film inplace.

Effective transmission (ET) can be measured using optical system 200, aschematic side-view of which is shown in FIG. 2. Optical system 200 iscentered on an optical axis 250 and includes a hollow lambertian lightbox that emits a lambertian light 215 through an emitting or exitsurface 212, a linear light absorbing polarizer 220, and a photodetector 230. Light box 210 is illuminated by a stabilized broadbandlight source 260 that is connected to an interior 280 of the light boxvia an optical fiber 270. A test sample the ET of which is to bemeasured by the optical system, is placed at location 240 between thelight box and the absorbing linear polarizer.

The ET of light redirecting film 100 can be measured by placing thelight redirecting film in location 240 with linear prisms 150 facing thephoto detector and microstructures 160 facing the light box. Next, thespectrally weighted axial luminance I₁ (luminance along optical axis250) is measured through the linear absorbing polarizer by the photodetector. Next, the light redirecting film is removed and the spectrallyweighted luminance I₂ is measured without the light redirecting filmplaced at location 240. ET is the ratio I₁/I₂. ET0 is the effectivetransmission when linear prisms 150 extend along a direction that isparallel to the polarizing axis of linear absorbing polarizer 220, andET90 is the effective transmission when linear prisms 150 extend along adirection that is perpendicular to the polarizing axis of the linearabsorbing polarizer. The average effective transmission (ETA) is theaverage of ET0 and ET90.

Effective transmission values disclosed herein were measured using aSpectraScan™ PR-650 SpectraColorimeter (available from Photo Research,Inc, Chatsworth, Calif.) for photo detector 230. Light box 210 was aTeflon cube with a total reflectance of about 85%.

In some cases, such as when light redirecting film 100 is used in adisplay system to increase the brightness and the linear prisms have anindex of refraction that is greater than about 1.6, the averageeffective transmission (ETA) of the light redirecting film is not lessthan about 1.5, or not less than about 1.55, or not less than about 1.6,or not less than about 1.65, or not less than about 1.7, or not lessthan about 1.75, or not less than about 1.8, or not less than about1.85.

Microstructures 160 in matte layer 140 are primarily designed to hideundesirable physical defects (such as, for example, scratches) and/oroptical defects (such as, for example, undesirably bright or “hot” spotsfrom a lamp in a display or illumination system) with no, or very littleadverse, effect on the capabilities of the light redirecting film toredirect light and enhance brightness. In such cases, second majorsurface 120 has an optical haze that is not greater than about 5%, ornot greater than about 4.5%, or not greater than about 4%, or notgreater than about 3.5%, or not greater than about 3%, or not greaterthan about 2.5%, or not greater than about 2%, or not greater than about1.5%, or not greater than about 1%; and an optical clarity that is notgreater than about 85%, or not greater than about 80%, or not greaterthan about 75%, or not greater than about 70%, or not greater than about65%, or not greater than about 60%.

Optical haze, as used herein, is defined as the ratio of the transmittedlight that deviates from the normal direction by more than 4 degrees tothe total transmitted light. Haze values disclosed herein were measuredusing a Haze-Gard Plus haze meter (available from BYK-Gardiner, SilverSprings, Md.) according to the procedure described in ASTM D1003.Optical clarity, as used herein, refers to the ratio (T₁−T₂)/(T₁+T₂),where T₁ is the transmitted light that deviates from the normaldirection between 1.6 and 2 degrees from the normal direction, and T₂ isthe transmitted light that lies between zero and 0.7 degrees from thenormal direction. Clarity values disclosed herein were measured using aHaze-Gard Plus haze meter from BYK-Gardiner.

Microstructures 160 can be any type microstructures that may bedesirable in an application. In some cases, microstructures 160 can berecessions. For example, FIG. 3A is a schematic side-view of a mattelayer 310 that is similar to matte layer 140 and includes recessedmicrostructures 320. In some cases, microstructures 160 can beprotrusions. For example, FIG. 3B is a schematic side-view of a mattelayer 330 that is similar to matte layer 140 and includes protrudingmicrostructures 340.

In some cases, microstructures 160 form a regular pattern. For example,FIG. 4A is a schematic top-view of microstructures 410 that are similarto microstructures 160 and form a regular pattern in a major surface415. In some cases, microstructures 160 form an irregular pattern. Forexample, FIG. 4B is a schematic top-view of microstructures 420 that aresimilar to microstructures 160 and form an irregular pattern. In somecases, microstructures 160 form a pseudo-random pattern that appears tobe random.

In general, microstructures 160 can have any height and any heightdistribution. In some cases, the average height (that is, the averagemaximum height minus the average minimum height) of microstructures 160is not greater than about 5 microns, or not greater than about 4microns, or not greater than about 3 microns, or not greater than about2 microns, or not greater than about 1 micron, or not greater than about0.9 microns, or not greater than about 0.8 microns, or not greater thanabout 0.7 microns.

FIG. 5 is a schematic side view of a portion of matte layer 140. Inparticular, FIG. 5 shows a microstructure 160 in major surface 120 andfacing major surface 142. Microstructure 160 has a slope distributionacross the surface of the microstructure. For example, themicrostructure has a slope θ at a location 510 where θ is the anglebetween normal line 520 which is perpendicular to the microstructuresurface at location 510 (α=90 degrees) and tangent line 530 which istangent to the microstructure surface at the same location. Slope θ isalso the angle between tangent line 530 and major surface 142 of thematte layer.

Optical haze and clarity of matte layer 140 were calculated using aprogram that was similar to commercially available ray tracing programssuch as, for example, TracePro (available from Lambda Research Corp.,Littleton, Mass.). In carrying out the calculations, it was assumed thateach microstructure had a Gaussian slope distribution with a half widthat half maximum (HWHM) equal to σ. It was further assumed that the mattelayer had an index of refraction equal to 1.5. The calculated resultsare shown in FIGS. 6 and 7. FIG. 6 is the calculated optical haze versussurface fraction “f” for nine different values of σ, where f is percentarea of major surface 120 covered by microstructures 160. FIG. 7 is thecalculated optical clarity versus f. In some cases, such as whenmicrostructures 160 effectively hide physical and/or optical defectswithout reducing or reducing very little the brightness, the pluralityof microstructures 160 covers at least about 70%, or at least about 75%,or at least about 80%, or at least about 85%, or at least about 90%, orat least about 95%, of second major surface 120. In some cases, such aswhen the microstructures have a Gaussian or normal slope distribution,the HWHM σ of the distribution is not greater than about 4.5 degrees, ornot greater than about 4 degrees, or not greater than about 3.5 degrees,or not greater than about 3 degrees, or not greater than about 2.5degrees, or not greater than about 2 degrees.

In the exemplary calculations disclosed above, it was assumed thatmicrostructures 160 have a Gaussian slope distribution with a HWHM equalto σ. In general, the microstructures can have any distribution that maybe desirable in an application. For example, in some cases, such as whenthe microstructures are spherical segments, the microstructures can havea uniform distribution between two limiting angles. Other exemplaryslope distributions include Lorentzian distributions, parabolicdistributions, and combinations of different, such as Gaussian,distributions. For example, in some cases, the microstructures can havea first Gaussian distribution with a smaller HWHM σ₁ added to, orcombined with, a second Gaussian distribution with a larger HWHM σ₂. Insome cases, the microstructures can have asymmetric slope distributions.In some cases, the microstructures can have symmetric distributions.

FIG. 8 is a schematic side-view of an optical film 800 that includes amatte layer 860 disposed on a substrate 850 similar to substrate 170.Matte layer 860 includes a first major surface 810 attached to substrate850, a second major surface 820 opposite the first major surface, and aplurality of particles 830 dispersed in a binder 840. Second majorsurface 820 includes a plurality of microstructures 870. A substantialportion, such as at least about 50%, or at least about 60%, or at leastabout 70%, or at least about 80%, or at least about 90%, ofmicrostructures 870 are disposed on and formed primarily because ofparticles 830. In other words, particles 830 are the primary reason forthe formation of microstructures 870. In such cases, particles 830 havean average size that is greater than about 0.25 microns, or greater thanabout 0.5 microns, or greater than about 0.75 microns, or greater thanabout 1 micron, or greater than about 1.25 microns, or greater thanabout 1.5 microns, or greater than about 1.75 microns, or greater thanabout 2 microns.

In some cases, matte layer 140 can be similar to matte layer 860 and caninclude a plurality of particles that are the primary reason for theformation of microstructures 160 in second major surface 120.

Particles 830 can be any type particles that may be desirable in anapplication. For example, particles 830 may be made of polymethylmethacrylate (PMMA), polystyrene (PS), or any other material that may bedesirable in an application. In general, the index of refraction ofparticles 830 is different than the index of refraction of binder 840,although in some cases, they may have the same refractive indices. Forexample, particles 830 can have an index of refraction of about 1.35, orabout 1.48, or about 1.49, or about 1.50, and binder 840 can have anindex of refraction of about 1.48, or about 1.49, or about 1.50.

In some cases, matte layer 140 does not include particles. In somecases, matte layer 140 includes particles, but the particles are not theprimary reason for the formation of microstructures 160. For example,FIG. 9 is a schematic side-view of an optical film 900 that includes amatte layer 960 similar to matter layer 140 disposed on a substrate 950similar to substrate 170. Matte layer 960 includes a first major surface910 attached to substrate 950, a second major surface 920 opposite thefirst major surface, and a plurality of particles 930 dispersed in abinder 940. Second major surface 970 includes a plurality ofmicrostructures 970. Even though matte layer 960 includes particles 930,the particles are not the primary reason for the formation ofmicrostructures 970. For example, in some cases, the particles are muchsmaller than the average size of the microstructures. In such cases, themicrostructures can be formed by, for example, microreplicating astructured tool. In such cases, the average size of particles 930 isless than about 0.5 microns, or less than about 0.4 microns, or lessthan about 0.3 microns, or less than about 0.2 microns, or less thanabout 0.1 microns. In such cases, a substantial fraction, such as atleast about 50%, or at least about 60%, or at least about 70%, or atleast about 80%, or at least about 90%, of microstructures 970 are notdisposed on particles that have an average size that is greater thanabout 0.5 microns, or greater than about 0.75 microns, or greater thanabout 1 micron, or greater than about 1.25 microns, or greater thanabout 1.5 microns, or greater than about 1.75 microns, or greater thanabout 2 microns. In some cases, the average size of particles 930 isless than the average size of microstructures 930 by at least a factorof about 2, or at least a factor of about 3, or at least a factor ofabout 4, or at least a factor of about 5, or at least a factor of about6, or at least a factor of about 7, or at least a factor of about 8, orat least a factor of about 9, or at least a factor of about 10. In somecases, if matte layer 960 includes particles 930, then matte layer 960has an average thickness “t” that is greater than the average size ofthe particles by at least about 0.5 microns, or at least about 1 micron,or at least about 1.5 microns, or at least about 2 microns, or at leastabout 2.5 microns, or at least about 3 microns. In some cases, if thematte layer includes a plurality of particles then the average thicknessof the matte layer is greater than the average thickness of theparticles by at least a factor of about 2, or at least a factor of about3, or at least a factor of about 4, or at least a factor of about 5, orat least a factor of about 6, or at least a factor of about 7, or atleast a factor of about 8, or at least a factor of about 9, or at leasta factor of about 10.

Referring back to FIG. 1, in some cases, light redirecting film 100 hassmall particles in at least some of the layers, such as prism layer 130,substrate layer 170, or matte layer 140, for increasing the index ofrefraction of the layer. For example, one or more layers in lightredirecting film 100 can include inorganic nanoparticles such as silicaor zirconia nanoparticles discussed in, for example U.S. Pat. No.7,074,463 (Jones et al.) and U.S. Patent Publication No. 2006/0210726.In some cases, light redirecting film 100 does not include any particleshaving an average size that is greater than about 2 microns, or about1.5 microns, or about 1 micron, or about 0.75 microns, or about 0.5microns, or about 0.25 microns, or about 0.2 microns, or about 0.15microns, or about 0.1 microns.

Microstructures 160 can be made using any fabrication method that may bedesirable in an application. For example, the microstructures can befabricated using microreplication from a tool, where the tool may befabricated using any available fabrication method, such as by usingengraving or diamond turning. Exemplary diamond turning systems andmethods can include and utilize a fast tool servo (FST) as described in,for example, PCT Published Application No. WO 00/48037, and U.S. Pat.Nos. 7,350,442 and 7,328,638, the disclosures of which are incorporatedin their entireties herein by reference thereto.

FIG. 10 is a schematic side-view of a cutting tool system 1000 that canbe used to cut a tool which can be microreplicated to producemicrostructures 160 and matte layer 140. Cutting tool system 1000employs a thread cut lathe turning process and includes a roll 1010 thatcan rotate around and/or move along a central axis 1020 by a driver1030, and a cutter 1040 for cutting the roll material. The cutter ismounted on a servo 1050 and can be moved into and/or along the rollalong the x-direction by a driver 1060. In general, cutter 1040 ismounted normal to the roll and central axis 1020 and is driven into theengraveable material of roll 1010 while the roll is rotating around thecentral axis. The cutter is then driven parallel to the central axis toproduce a thread cut. Cutter 1040 can be simultaneously actuated at highfrequencies and low displacements to produce features in the roll thatwhen microreplicated result in microstructures 160.

Servo 1050 is a fast tool servo (FTS) and includes a solid statepiezoelectric (PZT) device, often referred to as a PZT stack, whichrapidly adjusts the position of cutter 1040. FTS 1050 allows for highlyprecise and high speed movement of cutter 1040 in the x-, y- and/orz-directions, or in an off-axis direction. Servo 1050 can be any highquality displacement servo capable of producing controlled movement withrespect to a rest position. In some cases, servo 1050 can reliably andrepeatably provide displacements in a range from 0 to about 20 micronswith about 0.1 micron or better resolution.

Driver 1060 can move cutter 1040 along the x-direction parallel tocentral axis 1020. In some cases, the displacement resolution of driver1060 is better than about 0.1 microns, or better than about 0.01microns. Rotary movements produced by driver 1030 are synchronized withtranslational movements produced by driver 1060 to accurately controlthe resulting shapes of microstructures 160.

The engraveable material of roll 1010 can be any material that iscapable of being engraved by cutter 1040. Exemplary roll materialsinclude metals such as copper, various polymers, and various glassmaterials.

Cutter 1040 can be any type of cutter and can have any shape that may bedesirable in an application. For example, FIG. 11A is a schematicside-view of a cutter 1110 that has an arc-shape cutting tip 1115 with aradius “R”. In some cases, the radius R of cutting tip 1115 is at leastabout 100 microns, or at least about 150 microns, or at least about 200microns, or at least about 300 microns, or at least about 400 microns,or at least about 500 microns, or at least about 1000 microns, or atleast about 1500 microns, or at least about 2000 microns, or at leastabout 2500 microns, or at least about 3000 microns.

As another example, FIG. 11B is a schematic side-view of a cutter 1120that has a V-shape cutting tip 1125 with an apex angle β. In some cases,the apex angle β of cutting tip 1125 is at least about 100 degrees, orat least about 110 degrees, or at least about 120 degrees, or at leastabout 130 degrees, or at least about 140 degrees, or at least about 150degrees, or at least about 160 degrees, or at least about 170 degrees.As yet other examples, FIG. 11C is a schematic side-view of a cutter1130 that has a piece-wise linear cutting tip 1135, and FIG. 11D is aschematic side-view of a cutter 1140 that has a curved cutting tip 1145.

Referring back to FIG. 10, the rotation of roll 1010 along central axis1020 and the movement of cutter 1040 along the x-direction while cuttingthe roll material defines a thread path around the roll that has a pitchP₁ along the central axis. As cutter moves along a direction normal tothe roll surface to cut the roll material, the width of the material cutby the cutter changes as the cutter moves or plunges in and out.Referring to, for example FIG. 11A, the maximum penetration depth by thecutter corresponds to a maximum width P₂ cut by the cutter. In somecases, such as when microstructures 160 in light redirecting film 100are geometrically symmetric and sufficiently capable of hiding ormasking physical and/or optical defects without reducing, or reducingvery little, the brightness, the ratio P₂/P₁ is in a range from about1.5 to about 6, or from about 2 to about 5, or from about 2.5 to about4.

Several samples having microstructures similar to microstructures 160were made using a cutting tool system similar to cutting tool system1000 to make a patterned roll and subsequently microreplicating thepatterned tool to make matte layers similar to matter layer 140. FIG. 12is a top-view optical micrograph of a sample that was made using acutter similar to cutter 1110 where the radius of the cutting tip 1115was about 480 microns. The sample was geometrically symmetric and had asymmetric slope distribution, where by geometrically symmetric it ismeant that the average microstructure size along one direction, such asthe x-direction, is substantially the same as the average microstructuresize along an orthogonal direction, such as the y-direction. Inparticular, the sample had an average slope magnitude of about 1.18degrees along the x-direction and an average slope magnitude of about1.22 degrees along the y-direction. FIG. 13 is a top-view opticalmicrograph of a sample that was made using a cutter similar to cutter1110 where the radius of the cutting tip 1115 was about 480 microns. Thesample was geometrically symmetric and had an asymmetric slopedistribution. In particular, the sample had an average slope magnitudeof about 0.72 degrees along the x-direction and an average slopemagnitude of about 1.11 degrees along the y-direction. FIG. 14 is atop-view optical micrograph of a sample that was made using a cuttersimilar to cutter 1110 where the radius of the cutting tip 1115 wasabout 3300 microns. The sample was geometrically asymmetric and had anasymmetric slope distribution. In particular, the sample had an averageslope magnitude of about 0.07 degrees along the x-direction and anaverage slope magnitude of about 1.48 degrees along the y-direction.FIG. 15 is a top-view optical micrograph of a sample that was made usinga cutter similar to cutter 1110 where the radius of the cutting tip 1115was about 3300 microns. The sample was geometrically asymmetric and hadan asymmetric slope distribution. In particular, the sample had anaverage slope magnitude of about 0.18 degrees along the x-direction andan average slope magnitude of about 0.85 degrees along the y-direction.FIG. 16 is a top-view optical micrograph of a sample that was made usinga cutter similar to cutter 1120 where the apex angle of the cutting tip1125 was about 176 degrees. The sample was geometrically symmetric andhad a symmetric slope distribution. In particular, the sample had anaverage slope magnitude of about 2.00 degrees along the x-direction andan average slope magnitude of about 1.92 degrees along the y-direction.FIG. 17 is a top-view optical micrograph of a sample that was made usinga cutter similar to cutter 1120 where the apex angle of the cutting tip1125 was about 175 degrees. The sample was geometrically asymmetric andhad a symmetric slope distribution. In particular, the sample had anaverage slope magnitude of about 2.50 degrees along the x-direction andan average slope magnitude of about 2.54 degrees along the y-direction.

FIGS. 18A-18C are top-view scanning electron micrographs (SEMs) of asample at three different magnifications. The sample was made using acutter similar to cutter 1120 where the apex angle of the cutting tip1125 was about 176 degrees. The sample was geometrically symmetric.Using confocal microscopy, the average height of the microstructures wasmeasured to be about 2.67 microns. FIGS. 19A-19C are top-view SEMs of asample at three different magnifications. The sample was made using acutter similar to cutter 1110 where the radius of the cutting tip 1115was about 480 microns. The sample was geometrically symmetric. Usingconfocal microscopy, the average height of the microstructures wasmeasured to be about 2.56 microns. FIGS. 20A-20C are top-view SEMs of asample at three different magnifications. The sample was made using acutter similar to cutter 1110 where the radius of the cutting tip 1115was about 3300 microns. The sample was geometrically asymmetric. Usingconfocal microscopy, the average height of the microstructures wasmeasured to be about 1.46 microns.

The surfaces of a number of samples fabricated using the processoutlined above were characterized over an area of about 200 microns byabout 200 microns using atomic force microscopy (AFM). FIG. 21 is anexemplary AFM surface profile of one such sample, labeled sample A. Thesample had an optical transmission of about 94.9%, an optical haze ofabout 1.73%, and an optical clarity of about 79.5%. FIGS. 22A and 22Bare exemplary cross-sectional profiles of sample A along the x- andy-directions, respectively. FIG. 23 shows the percent slope distributionalong the x- and y-directions for sample A. Slopes S_(x) and S_(y) alongrespective x- and y-directions were calculated from the following twoexpressions:

$\begin{matrix}{S_{x} = \frac{\partial{H\left( {x,y} \right)}}{\partial x}} & (1) \\{S_{y} = \frac{\partial{H\left( {x,y} \right)}}{\partial y}} & (2)\end{matrix}$where H(x,y) is the surface profile. The slopes S_(x) and S_(y) werecalculated using a slope bin size of 0.5 degrees. As is evident fromFIG. 23, sample A had a symmetric slope distribution along both the x-and the y-directions. Sample A had a broader slope distribution alongthe x-direction and a narrower slope distribution along the y-direction.FIG. 24 shows the percent height distribution across the analyzedsurface for sample A. As is evident from FIG. 24, the sample had asubstantially symmetric height distribution relative to the peak heightof the sample which was about 4.7 microns. FIG. 25 shows the percentslope magnitude for sample A, where the slope magnitude S_(m) wascalculated from the following expression:

$\begin{matrix}{S_{m} = \sqrt{\left\lbrack \frac{\partial H}{\partial x} \right\rbrack^{2} + \left\lbrack \frac{\partial H}{\partial y} \right\rbrack^{2}}} & (3)\end{matrix}$

FIG. 26 shows the percent cumulative slope distribution S_(c)(θ) forsample A, where S_(c)(θ) was calculated from the following expression:

$\begin{matrix}{{S_{c}(\theta)} = \frac{{\int_{\theta}^{\infty}S_{m}}\ }{\int_{0}^{\infty}S_{m}}} & (4)\end{matrix}$

As is evident from FIG. 26, about 100% of the surface of sample A had aslope magnitude less than about 3.5 degrees. Furthermore, about 52% ofthe analyzed surface had slope magnitudes less than about 1 degree, andabout 72% of the analyzed surface had slope magnitudes less than about1.5 degrees.

Three additional samples similar to sample A, and labeled B, C, and D,were characterized as previously outlined. All four samples A-D hadmicrostructures similar to microstructures 160 and were made using acutting tool system similar to cutting tool system 1000 to make apatterned roll using a cutter similar to cutter 1120 and subsequentlymicroreplicating the patterned tool to make matte layers similar tomatter layer 140. Sample B had an optical transmittance of about 95.2%,an optical haze of about 3.28% and an optical clarity of about 78%;Sample C had an optical transmittance of about 94.9%, an optical haze ofabout 2.12%, and an optical clarity of about 86.1%; and sample D had anoptical transmittance of about 94.6%, an optical haze of about 1.71%,and an optical clarity of about 84.8%. In addition, six comparativesamples, labeled R1-R6, were characterized. Samples R1-R3 were similarto matte layer 860 and included a plurality of large beads dispersed ina binder, where the matte surfaces were primarily formed because of thebeads. Sample R1 had an optical haze of about 17.8% and an opticalclarity of about 48.5%, sample R2 (available from Dai Nippon PrintingCo., Ltd.) had an optical haze of about 32.2% and an optical clarity ofabout 67.2%, and sample R3 had an optical haze of about 4.7% and anoptical clarity of about 73.3%. Sample R4 was an embossed polycarbonatefilm (available from Keiwa Inc., Osaka, Japan) and had an optical hazeof about 23.2% and an optical clarity of about 39.5%.

FIG. 27 is the percent cumulative slope distribution S_(c)(θ) forsamples A-D and R1-R4. Each of samples A-D was similar to matte layer140 and included a structured major surface similar to structured majorsurface 120. As evident from FIG. 27, no more than about 7%, or about6.5%, or about 6%, or about 5.5%, or about 5%, or about 4.5%, or about4%, or about 3.5%, or about 3%, of the structured major surfaces of all,or at least some, of the samples A-D had a slope magnitude greater thanabout 3.5 degrees. Furthermore, no more than about 4%, or about 3.5%, orabout 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, orabout 0.9%, or about 0.8%, of the structured major surfaces of all, orat least some, of the samples A-D had a cumulative slope magnitudegreater than about 5 degrees.

Referring back to FIG. 1, when used in an optical system such as in aliquid crystal display, light redirecting film 100 is capable of hidingor masking optical and/or physical defects of the display and enhancingthe brightness of the display. In some cases, the average effectivetransmission of light redirecting film 100 is less by no more than about2%, or by no more than about 1.5%, or by no more than about 1%, or by nomore than about 0.75%, or by no more than about 0.5%, as compared to alight redirecting film that has the same construction as lightredirecting film 100, except for having a smooth second major surface120. In some cases, the average effective transmission of the lightredirecting film is greater than by no less than about 0.2%, or about0.3%, or about 0.4%, or about 0.5%, or about 1%, or about 1.5%, or about2%, ac compared to a light redirecting film that has the sameconstruction, except for having a smooth second major surface. As anexample, a light redirecting film was fabricated that was similar tolight redirecting film 100. Linear prisms 150 had a pitch of about 24microns, an apex angle 152 of about 90 degrees, and index of refractionof about 1.65. Second major surface 120 had an optical haze of about1.5% and an optical clarity of about 83%. The light redirecting film hadan average effective transmission of about 1.803. For comparison, asimilar light redirecting film that had the same construction (includingmaterial composition) except for comprising a smooth second majorsurface, had an average effective transmission of about 1.813.

As another example, a light redirecting film was fabricated that wassimilar to light redirecting film 100. Microstructures 160 were made byreplication from a tool that was cut with a cutter similar to cutter1110 where the radius of cutter tip 1115 was about 3300 microns. Linearprisms 150 had a pitch of about 24 microns, an apex angle 152 of about90 degrees, and index of refraction of about 1.567. Second major surface120 had an optical haze of about 1.71% and an optical clarity of about84.8%. The light redirecting film had an average effective transmissionof about 1.633. For comparison, a similar light redirecting film thathad the same construction (including material composition) except forcomprising a smooth second major surface, had an average effectivetransmission of about 1.626. Hence, the structured second major surface120 provided additional gain by increasing the average effectivetransmission by about 0.43%.

As another example, a light redirecting film was fabricated that wassimilar to light redirecting film 100. Microstructures 160 were made byreplication from a tool that was cut with a cutter similar to cutter1110 where the radius of cutter tip 1115 was about 4400 microns. Linearprisms 150 had a pitch of about 24 microns, an apex angle 152 of about90 degrees, and index of refraction of about 1.567. Second major surface120 had an optical haze of about 1.49% and an optical clarity of about82.7%. The light redirecting film had an average effective transmissionof about 1.583. For comparison, a similar light redirecting film thathad the same construction (including material composition) except forcomprising a smooth second major surface, had an average effectivetransmission of about 1.578. Hence, the structured second major surface120 provided additional gain by increasing the average effectivetransmission by about 0.32%.

As yet another example, a light redirecting film was fabricated that wassimilar to light redirecting film 100. Microstructures 160 were made byreplication from a tool that was cut with a cutter similar to cutter1110 where the radius of cutter tip 1115 was about 3300 microns. Linearprisms 150 had a pitch of about 24 microns, an apex angle 152 of about90 degrees, and index of refraction of about 1.567. Second major surface120 had an optical haze of about 1.35% and an optical clarity of about85.7%. The light redirecting film had an average effective transmissionof about 1.631. For comparison, a similar light redirecting film thathad the same construction (including material composition) except forcomprising a smooth second major surface, had an average effectivetransmission of about 1.593. Hence, the structured second major surface120 provided additional gain by increasing the average effectivetransmission by about 2.38%.

Substrate layer 170 can be or include any material that may be suitablein an application, such as a dielectric, a semiconductor, or a metal.For example, substrate layer 170 can include or be made of glass andpolymers such as polyethylene terephthalate (PET), polycarbonates, andacrylics. Substrate 170 can be rigid or flexible. Substrate 170 can haveany thickness and/or index of refraction that may be desirable in anapplication. For example, in some cases, substrate layer 170 can be PETand have a thickness of about 50 microns or about 175 microns.

FIG. 28 is a schematic side-view of an optical stack 2800 that includesa first light redirecting film 2805 disposed on a second lightredirecting film 2855. One or both of the light redirecting films can besimilar to light redirecting film 100. First light redirecting film 2805includes a first major surface 2810 and an opposing second major surface2815. The first major surface includes a first plurality ofmicrostructures 2820 that extend along the y-direction, and the secondmajor surface includes a second plurality of microstructures 2825.Second light redirecting film 2855 includes a third major surface 2860and an opposing fourth major surface 2865. Third major surface 2860faces second major surface 2815 of the first light redirecting film andincludes a third plurality of microstructures 2870 that extend along adifferent direction than the y-direction, such as the x-direction.Fourth major surface 2865 includes a fourth plurality of microstructures2875.

In some cases, first light redirecting film 2805 includes a matte layer2880 that includes second major surface 2815. Similarly, in some cases,second light redirecting film 2855 includes a matte layer 2885 thatincludes fourth major surface 2865.

In some cases, such as when optical stack 2800 is included in thebacklight of a liquid crystal display, linear microstructures 2820and/or 2870 can give rise to moiré. In some cases, the two lightredirecting films, and in particular, the top light redirecting film,can give rise to color mura. Color mura is due to the index dispersionof the light redirecting films. The first order color mura is typicallyvisible close to the viewing angle limit of the light redirecting filmwhile higher order color mura are typically visible at higher angles. Insome cases, such as when major surfaces 2815 and 2865 have sufficientlylow optical haze and clarity, the optical stack can effectively mask oreliminate moiré and color mura without significantly reducing thedisplay brightness. In such cases, each of the second and fourth majorsurfaces has an optical haze that is not greater than about 5%, or notgreater than about 4.5%, or not greater than about 4%, or not greaterthan about 3.5%, or not greater than about 3%, or not greater than about2.5%, or not greater than about 2%, or not greater than about 1.5%, ornot greater than about 1%; and each of the second and fourth majorsurfaces has an optical clarity that is not greater than about 85%, ornot greater than about 80%, or not greater than about 75%, or notgreater than about 70%, or not greater than about 65%, or not greaterthan about 60%.

In some cases, such as when optical stack 2800 is used in a displaysystem to increase the brightness, the average effective transmission(ETA) of the optical stack is not less than about 2.4, or not less thanabout 2.45, or not less than about 2.5, or not less than about 2.55, ornot less than about 2.6, or not less than about 2.65, or not less thanabout 2.7, or not less than about 2.75, or not less than about 2.8. Insome cases, the average effective transmission of optical stack 2800 isless by no more than about 1%, or about 0.75%, or about 0.5%, or about0.25%, or about 0.1%, as compared to an optical stack that has the sameconstruction (including material composition) except for comprisingsmooth second and fourth major surfaces. In some cases, the averageeffective transmission of optical stack 2800 is not less as compared toan optical stack that has the same construction except for having smoothsecond and fourth major surfaces. In some cases, the average effectivetransmission of optical stack 2800 is greater by at least about 0.1%, orabout 0.2%, or about 0.3%, as compared to an optical stack that has thesame construction except for comprising smooth second and fourth majorsurfaces. As an example, an optical stack was fabricated that wassimilar to optical stack 2800 and had an average effective transmissionof about 2.773. Each of respective second and fourth major surfaces 2815and 2865 had an optical haze of about 1.5% and an optical clarity ofabout 83%. The linear prisms had an index of refraction of about 1.65.For comparison, a similar optical stack that had the same constructionexcept for including smooth second and fourth major surfaces, had anaverage effective transmission of about 2.763. Hence, the structuredbottom major surfaces 2815 and 2865 provided additional again byincreasing the average effective transmission by about 0.36%.

As another example, an optical stack was fabricated that was similar tooptical stack 2800 and had an average effective transmission of about2.556. Each of respective second and fourth major surfaces 2815 and 2865had an optical haze of about 1.29% and an optical clarity of about86.4%. The linear prisms had a pitch of about 24 microns, an apex angleof about 90 degrees, and an index of refraction of about 1.567. Forcomparison, a similar optical stack that had the same constructionexcept for including smooth second and fourth major surfaces, had anaverage effective transmission of about 2.552. Hence, the structuredbottom major surfaces 2815 and 2865 provided additional again byincreasing the average effective transmission by about 0.16%.

As yet another example, an optical stack was fabricated that was similarto optical stack 2800 and had an average effective transmission of about2.415. Each of respective second and fourth major surfaces 2815 and 2865had an optical haze of about 1.32% and an optical clarity of about84.8%. The linear prisms had a pitch of about 24 microns, an apex angleof about 90 degrees, and an index of refraction of about 1.567. Forcomparison, a similar optical stack that had the same constructionexcept for including smooth second and fourth major surfaces, had anaverage effective transmission of about 2.404. Hence, the structuredbottom major surfaces 2815 and 2865 provided additional again byincreasing the average effective transmission by about 0.46%.

FIG. 29 is a schematic side-view of a display system 2900 for displayinginformation to a viewer 2999. The display system includes a liquidcrystal panel 2910 that is disposed on and is illuminated by a backlight2920. Liquid crystal panel 2910 includes a liquid crystal cell 2930 thatis disposed between linear light absorbing polarizers 2935 and 2940. Insome cases, such as when display system 2900 displays an image to viewer2999, liquid crystal panel 2910 can be pixilated.

Backlight 2920 includes a lightguide 2970 that receives light through anedge of the lightguide from a lamp 2990 that is housed in a sidereflector 2995, a back reflector 2980 for reflecting light that isincident on the back reflector toward viewer 2999, an optical diffuser2960 for homogenizing light 2985 that exits from an emitting surface2975 of the lightguide, and optical stack 2800 from FIG. 28 disposedbetween the optical diffuser and a reflective polarizer 2950.

Optical stack 2800 includes light redirecting films 2805 and 2855. Insome cases, linear prisms of the two light redirecting films areorthogonally oriented relative to each other. For example, linear prisms2820 can extend along the y-direction and linear prisms 2870 can beoriented along the x-direction. Microstructures 2825 and 2875 facelightguide 2970 and prismatic microstructures 2820 and 2870 face awayfrom the lightguide.

Optical stack 2800 enhances the brightness, such as the on-axisbrightness, of the display system. At the same time, respective secondand fourth major surfaces 2815 and 2865 of the optical stack havesufficiently low optical clarities to mask physical defects such asscratches, and hide and/or eliminate optical defects such moiré andcolor mura.

Reflective polarizer 2950 substantially reflects light that has a firstpolarization state and substantially transmits light that has a secondpolarization state, where the two polarization states are mutuallyorthogonal. For example, the average reflectance of reflective polarizer2950 in the visible for the polarization state that is substantiallyreflected by the reflective polarizer is at least about 50%, or at leastabout 60%, or at least about 70%, or at least about 80%, or at leastabout 90%, or at least about 95%. As another example, the averagetransmittance of reflective polarizer 2950 in the visible for thepolarization state that is substantially transmitted by the reflectivepolarizer is at least about 50%, or at least about 60%, or at leastabout 70%, or at least about 80%, or at least about 90%, or at leastabout 95%, or at least about 97%, or at least about 98%, or at leastabout 99%. In some cases, reflective polarizer 2950 substantiallyreflects light having a first linear polarization state (for example,along the x-direction) and substantially transmits light having a secondlinear polarization state (for example, along the y-direction).

Any suitable type of reflective polarizer may be used for reflectivepolarizer layer 2950 such as, for example, a multilayer optical film(MOF) reflective polarizer, a diffusely reflective polarizing film(DRPF) having a continuous phase and a disperse phase, such as aVikuiti™ Diffuse Reflective Polarizer Film (“DRPF”) available from 3MCompany, St. Paul, Minn., a wire grid reflective polarizer described in,for example, U.S. Pat. No. 6,719,426, or a cholesteric reflectivepolarizer.

For example, in some cases, reflective polarizer 2950 can be or includean MOF reflective polarizer, formed of alternating layers of differentpolymer materials, where one of the sets of alternating layers is formedof a birefringent material, where the refractive indices of thedifferent materials are matched for light polarized in one linearpolarization state and unmatched for light in the orthogonal linearpolarization state. In such cases, an incident light in the matchedpolarization state is substantially transmitted through reflectivepolarizer 2950 and an incident light in the unmatched polarization stateis substantially reflected by reflective polarizer 2950. In some cases,an MOF reflective polarizer 2950 can include a stack of inorganicdielectric layers.

As another example, reflective polarizer 2950 can be or include apartially reflecting layer that has an intermediate on-axis averagereflectance in the pass state. For example, the partially reflectinglayer can have an on-axis average reflectance of at least about 90% forvisible light polarized in a first plane, such as the xy-plane, and anon-axis average reflectance in a range from about 25% to about 90% forvisible light polarized in a second plane, such as the xz-plane,perpendicular to the first plane. Such partially reflecting layers aredescribed in, for example, U.S. Patent Publication No. 2008/064133, thedisclosure of which is incorporated herein in its entirety by reference.

In some cases, reflective polarizer 2950 can be or include a circularreflective polarizer, where light circularly polarized in one sense,which may be the clockwise or counterclockwise sense (also referred toas right or left circular polarization), is preferentially transmittedand light polarized in the opposite sense is preferentially reflected.One type of circular polarizer includes a cholesteric liquid crystalpolarizer.

In some cases, reflective polarizer 2950 can be a multilayer opticalfilm that reflects or transmits light by optical interference, such asthose described in Provisional U.S. Patent Application No. 61/116,132,filed Nov. 19, 2009; Provisional U.S. Patent Application No. 61/116,291,filed Nov. 19, 2008; Provisional U.S. Patent Application No. 61/116,294,filed Nov. 19, 2008; Provisional U.S. Patent Application No. 61/116,295,filed Nov. 19, 2008; Provisional U.S. Patent Application No. 61/116,295,filed Nov. 19, 2008; and International Patent Application No. PCT/US2008/060311, filed May 19, 2008, claiming priority from Provisional U.S.Patent Application No. 60/939,085, filed Apr. 15, 200; all incorporatedherein by reference in their entirety.

Optical diffuser 2960 has the primary functions of hiding or maskinglamp 2990 and homogenizing light 2985 that is emitted by lightguide2970. Optical diffuser 2960 has a high optical haze and/or a highdiffuse optical reflectance. For example, in some cases, the opticalhaze of the optical diffuser is not less than about 40%, or not lessthan about 50%, or not less than about 60%, or not less than about 70%,or not less than about 80%, or not less than about 85%, or not less thanabout 90%, or not less than about 95%. As another example, the diffuseoptical reflectance of the optical diffuser is not less than about 30%,or not less than about 40%, or not less than about 50%, or not less thanabout 60%.

Optical diffuser 2960 can be or include any optical diffuser that may bedesirable and/or available in an application. For example, opticaldiffuser 2960 can be or include a surface diffuser, a volume diffuser,or a combination thereof. For example, optical diffuser 2960 can includea plurality of particles having a first index of refraction n₁ dispersedin a binder or host medium having a different index of refraction n₂,where the difference between the two indices of refraction is at leastabout 0.01, or at least about 0.02, or at least about 0.03, or at leastabout 0.04, or at least about 0.05.

Back reflector 2980 receives light that is emitted by the lightguideaway from viewer 2999 along the negative z-direction and reflects thereceived light towards the viewer. Display systems such as displaysystem 2900 where lamp 2990 is placed along an edge of a lightguide, aregenerally referred to as edge-lit or backlit displays or opticalsystems. In some cases, the back reflector can be partially reflectiveand partially transmissive. In some cases, the back reflector can bestructured, for example, have a structured surface.

Back reflector 2980 can be any type reflector that may be desirableand/or practical in an application. For example, the back reflector canbe a specular reflector, a semi-specular or semi-diffuse reflector, or adiffuse reflector, such as those disclosed in International PatentApplication No. PCT/US 2008/064115, filed May 19, 2008, claimingpriority from Provisional U.S. Patent Application No. 60/939,085, filedMay 20, 2007, both incorporated herein by reference in their entirety.For example, the reflector can be an aluminized film or a multi-layerpolymeric reflective film, such as an enhanced specular reflector (ESR)film (available from 3M Company, St. Paul, Minn.). As another example,back reflector 2980 can be a diffuse reflector having a whiteappearance.

As used herein, terms such as “vertical”, “horizontal”, “above”,“below”, “left”, “right”, “upper” and “lower”, “clockwise” and “counterclockwise” and other similar terms, refer to relative positions as shownin the figures. In general, a physical embodiment can have a differentorientation, and in that case, the terms are intended to refer torelative positions modified to the actual orientation of the device. Forexample, even if the image in FIG. 1 is flipped as compared to theorientation in the figure, first major surface 110 is still consideredto be the top major surface.

All patents, patent applications, and other publications cited above areincorporated by reference into this document as if reproduced in full.While specific examples of the invention are described in detail aboveto facilitate explanation of various aspects of the invention, it shouldbe understood that the intention is not to limit the invention to thespecifics of the examples. Rather, the intention is to cover allmodifications, embodiments, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

What is claimed is:
 1. A light redirecting film comprising: a firstmajor surface comprising a plurality of first microstructures extendingalong a first direction; and a second major surface opposite the firstmajor surface comprising a plurality of second microstructures, thesecond major surface having an optical haze that is not greater thanabout 3% and an optical clarity that is not greater than about 85%,wherein the light redirecting film has an average effective transmissionthat is not less than about 1.75.
 2. The light redirecting film of claim1, wherein the plurality of first microstructures comprises a pluralityof linear prisms extending along the first direction.
 3. The lightredirecting film of claim 1, wherein a maximum height of amicrostructure in the plurality of first microstructures is differentthan a maximum height of another microstructure in the plurality offirst microstructures.
 4. The light redirecting film of claim 1, whereina height of a microstructure in the plurality of first microstructuresvaries along the first direction.
 5. The light redirecting film of claim1, wherein the plurality of second microstructures comprises a pluralityof protrusions.
 6. The light redirecting film of claim 1, wherein theplurality of second microstructures comprises a plurality of recessions.7. The light redirecting film of claim 1, wherein the plurality ofsecond microstructures covers at least about 80% of the second majorsurface.
 8. The light redirecting film of claim 1, wherein the pluralityof second microstructures covers at least about 85% of the second majorsurface.
 9. The light redirecting film of claim 1, wherein the pluralityof second microstructures covers at least about 90% of the second majorsurface.
 10. The light redirecting film of claim 1, wherein theplurality of second microstructures covers at least about 95% of thesecond major surface.
 11. The light redirecting film of claim 1, whereinthe plurality of second microstructures forms a regular pattern.
 12. Thelight redirecting film of claim 1, wherein the plurality of secondmicrostructures forms an irregular pattern.
 13. The light redirectingfilm of claim 1, wherein no more than about 7% of the second majorsurface has a slope magnitude that is greater than about 3.5 degrees.14. The light redirecting film of claim 1, wherein no more than about 5%of the second major surface has a slope magnitude that is greater thanabout 3.5 degrees.
 15. The light redirecting film of claim 1, wherein nomore than about 3% of the second major surface has a slope magnitudethat is greater than about 3.5 degrees.
 16. The light redirecting filmof claim 1, wherein no more than about 4% of the second major surfacehas a slope magnitude that is greater than about 5 degrees.
 17. Thelight redirecting film of claim 1, wherein no more than about 2% of thesecond major surface has a slope magnitude that is greater than about 5degrees.
 18. The light redirecting film of claim 1, wherein no more thanabout 1% of the second major surface has a slope magnitude that isgreater than about 5 degrees.
 19. The light redirecting film of claim 1,wherein the second major surface has an optical haze that is not greaterthan about 2.5%.
 20. The light redirecting film of claim 1, wherein thesecond major surface has an optical haze that is not greater than about2%.
 21. The light redirecting film of claim 1, wherein the second majorsurface has an optical haze that is not greater than about 1.5%.
 22. Thelight redirecting film of claim 1, wherein the second major surface hasan optical haze that is not greater than about 1%.
 23. The lightredirecting film of claim 1, wherein the second major surface has anoptical clarity that is not greater than about 80%.
 24. The lightredirecting film of claim 1, wherein the second major surface has anoptical clarity that is not greater than about 75%.
 25. The lightredirecting film of claim 1, wherein the second major surface has anoptical clarity that is not greater than about 70%.
 26. The lightredirecting film of claim 1, wherein the average effective transmissionof the light redirecting film is not less than about 1.80.
 27. The lightredirecting film of claim 1, wherein the average effective transmissionof the light redirecting film is not less than about 1.85.
 28. The lightredirecting film of claim 1, wherein a substantial fraction of thesecond microstructures is not disposed on particles that have an averagesize of greater than about 0.5 microns.
 29. The light redirecting filmof claim 1 not comprising particles having an average size that isgreater than about 0.5 microns.
 30. The light redirecting film of claim1 comprising a plurality of particles having an average size that is notgreater than about 0.2 microns.
 31. The light redirecting film of claim1 comprising a plurality of particles having an average size that is notgreater than about 0.1 microns.
 32. The light redirecting film of claim1, wherein an average height of the plurality of second microstructuresis not greater than about 3 microns.
 33. The light redirecting film ofclaim 1, wherein an average height of the plurality of secondmicrostructures is not greater than about 2 microns.
 34. The lightredirecting film of claim 1, wherein an average height of the pluralityof second microstructures is not greater than about 1 micron.
 35. Thelight redirecting film of claim 1, wherein the microstructures in theplurality of second microstructures have a slope distribution having ahalf width half maximum (HWHM) that is not greater than about 6 degrees.36. The light redirecting film of claim 1, wherein the microstructuresin the plurality of second microstructures have a slope distributionhaving a half width half maximum (HWHM) that is not greater than about 5degrees.
 37. The light redirecting film of claim 1, wherein themicrostructures in the plurality of second microstructures have a slopedistribution having a half width half maximum (HWHM) that is not greaterthan about 4 degrees.
 38. The light redirecting film of claim 1comprising: a substrate layer having opposing first and second majorsurfaces; a first layer disposed on the first major surface of thesubstrate layer and comprising the first major surface of the lightredirecting film; and a matte layer disposed on the second major surfaceof the substrate layer and comprising the second major surface of thelight redirecting layer.
 39. The light redirecting film of claim 38,wherein the first layer has an index of refraction that is not less thanabout 1.5.
 40. The light redirecting film of claim 38, wherein the firstlayer has an index of refraction that is not less than about 1.55. 41.The light redirecting film of claim 38, wherein the first layer has anindex of refraction that is not less than about 1.6.
 42. The lightredirecting film of claim 38, wherein the first layer has an index ofrefraction that is not less than about 1.65.
 43. The light redirectingfilm of claim 38, wherein the first layer has an index of refractionthat is not less than about 1.7.
 44. The light redirecting film of claim38, wherein the first layer has an index of refraction that is in arange from about 1.5 to about 1.8.
 45. The light redirecting film ofclaim 38, wherein the matte layer has an index of refraction that is notless than about 1.4.
 46. The light redirecting film of claim 38, whereinthe matte layer has an index of refraction that is not less than about1.5.
 47. The light redirecting film of claim 38, wherein the matte layerhas an index of refraction that is in a range from about 1.4 to about1.6.
 48. The light redirecting film of claim 38, wherein the matte layercomprises a plurality of particles having an average size that is lessthan an average size of the plurality of second microstructures by atleast a factor of
 5. 49. The light redirecting film of claim 48, whereinthe plurality of particles have an average size that is less than theaverage size of the plurality of second microstructures by at least afactor of
 10. 50. The light redirecting film of claim 38, wherein if thematte layer comprises particles, then an average thickness of the mattelayer is at least 2 microns greater than an average size of theparticles.
 51. The light redirecting film of claim 38, wherein if thematte layer comprises particles, then an average thickness of the mattelayer is greater than an average size of the particles by at least afactor of
 2. 52. The light redirecting film of claim 38, wherein thesecond major surface has a slope distribution across the second majorsurface, the slope distribution having a half width at half maximum(HWHM) that is not greater than about 4 degrees.
 53. The lightredirecting film of claim 52, wherein the HWHM is not greater than about3.5 degrees.
 54. The light redirecting film of claim 52, wherein theHWHM is not greater than about 3 degrees.
 55. The light redirecting filmof claim 52, wherein the HWHM is not greater than about 2.5 degrees. 56.The light redirecting film of claim 52, wherein the HWHM is not greaterthan about 2 degrees.
 57. A backlight comprising: a light source; afirst light redirecting film according to claim 1 receiving light fromthe light source; and a second light redirecting film according to claim1 disposed on the first light redirecting film, wherein the firstdirection for the first light redirecting film is different than thefirst direction for the second light redirecting film.
 58. The backlightof claim 57, wherein the second major surface of each of the first andsecond light redirecting films faces the light source and the firstmajor surface of each of the first and second light redirecting filmsfaces away from the light source.
 59. A light redirecting filmcomprising: a first major surface comprising a plurality of linearmicrostructures; and a second major surface opposite the first majorsurface comprising a plurality of second microstructures, the secondmajor surface having an optical haze that is not greater than about 3%and an optical clarity that is not greater than about 85%, wherein anaverage effective transmission of the light redirecting film is not lessor is less by no more than about 1.5% as compared to a light redirectingfilm that has the same construction except for comprising a smoothsecond major surface.
 60. The light redirecting film of claim 59 havingan average effective transmission that is less by no more than about 1%as compared to a light redirecting film that has the same constructionexcept for comprising a smooth second major surface.
 61. The lightredirecting film of claim 59, wherein the plurality of secondmicrostructures comprises geometrical symmetry and asymmetric slopedistribution.
 62. The light redirecting film of claim 59, wherein theplurality of second microstructures comprises geometrical asymmetry andsymmetric slope distribution.
 63. An optical stack comprising: a firstlight redirecting film comprising a first major surface and an opposingsecond major surface, the first major surface comprising a firstplurality of microstructures extending along a first direction, thesecond major surface comprising a second plurality of microstructures;and a second light redirecting film comprising a third major surface andan opposing fourth major surface, the third major surface facing thesecond major surface of the first light redirecting film and comprisinga third plurality of microstructures extending along a second directiondifferent than the first direction, the fourth major surface comprisinga fourth plurality of microstructures, wherein: each of the second andfourth major surfaces has an optical haze that is not greater than about3% and an optical clarity that is not greater than about 85%.
 64. Theoptical stack of claim 63, wherein each of the second and fourth majorsurfaces has an optical haze that is not greater than about 2.5%. 65.The optical stack of claim 63, wherein each of the second and fourthmajor surfaces has an optical haze that is not greater than about 2%.66. The optical stack of claim 63, wherein each of the second and fourthmajor surfaces has an optical haze that is not greater than about 1.5%.67. The optical stack of claim 63, wherein each of the second and fourthmajor surfaces has an optical haze that is not greater than about 1%.68. The optical stack of claim 63, wherein each of the second and fourthmajor surfaces has an optical clarity that is not greater than about80%.
 69. The optical stack of claim 63, wherein each of the second andfourth major surfaces has an optical clarity that is not greater thanabout 75%.
 70. The optical stack of claim 63, wherein each of the secondand fourth major surfaces has an optical clarity that is not greaterthan about 70%.
 71. The optical stack of claim 63 having an averageeffective transmission that is not less than about 2.5.
 72. The opticalstack of claim 63 having an average effective transmission that is notless than about 2.6.
 73. The optical stack of claim 63 having an averageeffective transmission that is not less than about 2.7.
 74. An opticalstack comprising: a first light redirecting film comprising a firstmajor surface and an opposing second major surface, the first majorsurface comprising a first plurality of microstructures extending alonga first direction; and a second light redirecting film comprising athird major surface and an opposing fourth major surface, the thirdmajor surface facing the second major surface of the first lightredirecting film and comprising a third plurality of microstructuresextending along a second direction different than the first direction,wherein each of the second and fourth major surfaces has an optical hazethat is not greater than about 3% and an optical clarity that is notgreater than about 85%, and wherein an average effective transmission ofthe optical stack is not less or is less by no more than about 1% ascompared to an optical stack that has the same construction except forcomprising smooth second and fourth major surfaces.
 75. The opticalstack of claim 74 having an average effective transmission that is lessby no more than about 0.5% as compared to an optical stack that has thesame construction except for comprising smooth second and fourth majorsurfaces.
 76. The optical stack of claim 74 having an average effectivetransmission that is less by no more than about 0.25% as compared to anoptical stack that has the same construction except for comprisingsmooth second and fourth major surfaces.
 77. The optical stack of claim74 having an average effective transmission that is not less as comparedto an optical stack that has the same construction except for comprisingsmooth second and fourth major surfaces.